[erlang-questions] list:join() for erlang?

Bob Ippolito <>
Thu Sep 13 22:38:29 CEST 2007


On 9/13/07, David Mercer <> wrote:
> On Thursday, September 13, 2007, Ben Munat wrote:
> > Out of curiosity -- still feeling my way around erlang and functional
> > programming -- aren't the "helper" functions here stack-recursive (i.e.
> *not*
> > tail-recursive)?
>
> Looks like you're right.  Here's my tail-recursive version of the same:
>
>         join([], _) -> [];
>         join([S1 | S_Rest], Sep) ->
>                 S1 ++ reverse_join(lists:reverse(S_Rest), Sep, []).
>
>         reverse_join([], _, Joined) -> Joined;
>         reverse_join([S1 | S_Rest], Sep, Joined) ->
>                 reverse_join(S_Rest, Sep, Sep ++ S1 ++ Joined).

Using ++ all over the place is probably a bad idea, at least according
to the performance guide and my intuition. I haven't profiled it.

This is our join/2, which is largely the same but uses lists:flatten/1
instead of ++:

join([], _Separator) ->
    [];
join([S], _Separator) ->
    lists:flatten(S);
join(Strings, Separator) ->
    lists:flatten(revjoin(lists:reverse(Strings), Separator, [])).

revjoin([], _Separator, Acc) ->
    Acc;
revjoin([S | Rest], Separator, []) ->
    revjoin(Rest, Separator, [S]);
revjoin([S | Rest], Separator, Acc) ->
    revjoin(Rest, Separator, [S, Separator | Acc]).

-bob



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